This invention relates to an exhaust muffler for an internal combustion engine. The exhaust muffler of this invention can be applied to such internal combustion engine as an air-cooled two cycle gasoline engine and the like which is used for a portable working machine such as a brush cutter, chain saw and the like.
In the internal combustion engine as described above, exhaust gas is introduced into an exhaust muffler through an exhaust port of a cylinder, hazardous substances in exhaust gas are removed from exhaust gas and noise is reduced in the exhaust muffler, and then exhaust gas is discharged into atmosphere.
High decontamination efficiency of exhaust gas is required for such exhaust muffler as mentioned above. Particularly, in such internal combustion engine as an air-cooled two cycle gasoline engine and the like mounted on a working machine which is used with carrying by worker on or close to his body, reduction of hydrocarbon (HC), carbon monoxide (CO) and the like in exhaust gas is strongly required and also subjected to the legal restriction to protect worker's health.
Therefore, in order to improve the decontamination efficiency of exhaust gas, exhaust mufflers with following structure have been proposed.
For example, there is an exhaust muffler with structure inside of which provided with oxidation catalyst to reduce the content of hazardous substances such as HC, CO and the like in exhaust gas.
Another example of conventional exhaust muffler is structured to re-combust exhaust gas in the exhaust muffler by re-ignition on it discharged from the cylinder.
However, the size of exhaust muffler attached to the conventional portable working machine as mentioned above is small. Therefore, even though catalyst is provided in the exhaust muffler, the decontamination efficiency of exhaust gas by the catalyst was low due to smaller contact area of the catalyst with exhaust gas. In addition, if exhaust gas is re-combusted in the exhaust muffler, temperature of the surface of the exhaust muffler and of exhaust gas discharged to atmosphere through the exhaust muffler is risen up due to smaller size of the exhaust muffler, resulting in increased damage of the exhaust muffler and causing the hazard to worker being accidentally burnt during work carrying with it. Furthermore, it takes much trouble to change or maintain the catalyst.